Please don't do this yet. Sanding the build plate should always be a last resort!!!, I can't count the amount of people on the Photon group that sanded the build plate and had to buy a new one because they fucked it up. Re level with the paper method, it's the easiest and the most consistent, normal printer paper is fine. Try to find some print settings for the resin you are using and your printer and test them, use a small print dead centre in the plate, it'll be the most likely level spot. Keep playing until it sticks then try a bigger print, if it starts to fail further out on the plate, re level and try again. If you find prints still wont stick in some spots or just wont stick even after making the z height stupidly low, after that then maybe sanding the plate will be an option. However you must have a perfectly flat surface and keep the plate perfectly flat otherwise you could fuck the whole thing up and then you'll need to buy a new plate anyway. I cant stress this enough, do not sand a resin build plate unless you've exhausted all other options, unlike normal FDM you have very very little room for error and seeing most are aluminium it only takes a little to much pressure on one side to screw the plate up.